Add a "Test" Button for OIDC Configuration Validation on the Admin Page

[mastercactapus]

What problem would you like to solve? Please describe: Currently, when setting up OIDC integration for the first time, the only way to validate the configuration is to set it, log out (or open a new browser) and then test the login flow. This could be a tedious process especially when the configuration does not work as expected.

Describe the solution you'd like: A "Test" button should be added next to the OIDC issuer URL input field on the admin config page. This button would promptly validate the entered URL, running basic checks (for example, verifying the payload of .well-known/openid-configuration). This feature should:

• Be as helpful as possible
• Limited to admin users (permission wise)
• Setup as a mutation (1 test per click)

Describe alternatives you've considered: An alternative could be building an automatic configuration checker that runs these checks in the background every time a change is made, although this could potentially lead to unnecessary processing if the user is not ready for the check to occur.

Additional context: Additional enhancements to consider to further improve the user experience:

• On failure (e.g. 404), the system could attempt to search up the tree (up to x-number of parent paths) for a valid one and suggest it to the user.
• Look into the possibility of validating credentials as well.